Treating aluminum surfaces



Patented May 9, 1939 UNITED STATES PATENT YOFFICE TREATING ALULIINUM SURFACES No Drawing.

Application July 3, 1936,

Serial No. 88,828

4 Claims.

Our invention relates to the preparation of aluminum articles for various uses, and especially to the provisionof an improved surface or coating on aluminum articles. An object of our in- 5 vention is to provide aluminum articles subject to wear or friction with improved, substantially selflubricating surfaces, and to provide a process of producing such surfaces. The aluminum articles contemplated by the invention include screws,

10 weather strips, pistons, and threaded, rotating, or reciprocating parts or articles in general, as well as other aluminum articles which in use may be subjected to friction, and especially to frictional contact with other metal articles or surfaces.

15 It has long been realized that bare aluminum surfaces are not always well adapted for use in frictional contact with other metal surfaces because of the inherent softness of the metal and its tendency to bind or adhere, especially when in 20 contact with other bare aluminum surfaces. This diiiiculty can often be overcome by proper lubrication with suitable oils or greases, but such lubrication is sometimes diflicult or impossible to effect, and sometimes objectionable from the 25 standpoint of cleanliness -or appearance. The general desirability of aluminum and its alloys for many applications because of their lightness, strength and attractive appearance, however, has led to the proposal of various methods of improving the characteristics of such metals when subjected to wear or friction. For example, it has been proposed to provide contacting aluminum surfaces on threaded, parts, valves, etc., with oxide coatings to eliminate or minimize adhesion, seiz- 5 ing or binding, and considerable improvement has been obtained in this manner. It has also been proposed to provide moving parts, such as the pistons of internal combustion engines, with a coating of a metal harder than aluminum, such as 0 iron or nickel, but this proposal has not been accepted commercially. Another proposal has been to provide pistons with an oxide coating to resist sending or scoring, which are encountered with bare aluminum pistons under adverse lubricat- 45 ing conditions which occur principally when a motor is started at very low temperatures. The oxide coating has proven quite effective in eliminating this particular difliculty, but has not generally improved the wearing qualities of alumi- 50 num pistons. It may therefore be stated that some of the surface treatments or coatings for aluminum proposed or developed previous to our invention have been valuable contributions to the art, but none of them has been entirely satisfac- 55 tory in all respects, and none of them has combined high resistance to scuffing or other localized abrasion with exceptionally long life or wear resistance and substantially permanent self-lubrication. It is a further object of our invention to provide a coating or surface treatment which im- 5 parts all of these desired characteristics to aluminum articles of various forms and types. We have found that the desiderata referred to hereinabove are obtained to a remarkable extent and at reasonable cost if aluminum articles are provided with a permanent coating of substantially dry, finely divided graphite of suitable form. The lubricating properties of deflocculated graphite and some other finely divided graphites are well known, but to the best of our knowledge it has not been previously known that these finely divided graphites, in dry form, could be caused to adhere substantially permanently to aluminum articles, nor that when so applied they would markedly lengthen the service life of such articles and make them substantially self-lubricating over long periods.

We have found that these results are obtained if the graphite is applied in conjunction with suitable binding media, which are preferably inorganic in nature, or at least substantially so, and which are adapted to be united firmly with an aluminum article, preferably in the form of an adherent coating thereon, and to hold the graphite thereto substantially permanently without materially interfering with the lubricating effect of the graphite. The binding media which are most generally suitable are certain silicates, which may be used alone or in combination with certain oxide coatings, as will be described more fully hereinbelow.

Our invention is applicable to substantially all types of fabricated aluminum articles which are subjected to wear or friction, including castings, such as pistons for internal combustion engines; wrought products and formed sheet products, such as weather strip, for example; and articles shaped otherwise, suchas screw machine products. It is to be understood that the wordaluminum" is used herein and in the appended claims to include pure aluminum, commercial aluminum with the usual impurities, and aluminum base alloys, and that expressions such as aluminum pistons, for example, refer to pistons made from aluminum base alloys suitable for that purpose.

In the preferred embodiment of our invention, the aluminum article to be provided with a selflubricating surface is first provided with an oxide coating. The terms "oxide coating and aluminum oxide coating are used herein with the meaning common in the art to denote artificially produced adsorbent coatings of appreciable thickness composed in substantial part of aluminum oxide. Suitable coatings of this nature are most readily produced by treating the aluminum article electrolytically as an anode, using direct or alternating current, in a bath of suitable electrolyte. The electrolytes most suitable for this purpose are acids, including sulfuric acid, chromic acid, dibasic organic acids, and mixtures of acids, and several processes of producing oxide coatings by the use of electrolytes of this type are known to the art. Such processes can be controlled or modified to produce aluminum oxide coatings varying in hardness, thickness, and density or porosity, and a coating suitable as a base or binding medium for graphite is readily attainable.

After the oxide coating is applied to the aluminum article, it is impregnated or otherwise suitably treated with finely divided graphite, which is best applied in the form of a fluid dispersion, such as a suspension, or preferably a colloidalsolution or 501. The graphite should be very finely divided and in a form having good lubricating properties, such as the type known as "deflocculated graphite. Since a substantially dry coating or deposit of the graphite is desired on the final product, it should be dispersed in a volatile or drying vehicle, which may conveniently be a substantially aqueous vehicle, for example. Deflocculated graphite of suitable fineness is available on the market in the form of concentrated dispersions in various vehicles, but we have found that much better results for our purposes are generally obtained with dispersions in volatile vehicles, such as the colloidal-graphited water marketed as Aquadag, for example, than with dispersions in non-aqueous and relatively non-volatile vehicles of an organic nature, such as oils, greases and glycerin. In fact, graphite applied in the form of a substantially aqueous dispersion to an oxide-coated aluminum article has resulted in self-lubricating "surfaces which were quite adherent under conditions such that graphite applied in the form of dispersions in other vehicles was quickly removed.

Concentrated colloidal graphite dispersions, such as "Aquadag, contain up to about 22. per cent graphite, and may be applied to oxidecoated aluminum articles without dilution in accordance with our invention. It is generally preferable to employ a somewhat more dilute graphite dispersion, however, as application is thereby facilitated. Dispersions of suitable concentration may be obtained by mixing the concentrated "Aquadag with up to 8 volumes or more of water, and preferably with about 3 to '7 volumes of water. Otherwise stated, the graphite may be applied in the form of dispersions of about 2 per cent or less to about 22 per cent or more concentration, and preferably of about 2.5 to 8 or 10 per cent concentration. Such dispersions may be applied in any suitable manner, as by immersing therein the article to be coated, or painting or rubbing them onto the surface of the article with a brush or cloth, for example. After application of the dispersion, the article is dried at a suitable temperature, up to C., for example, to drive off the water or other vehicle and leave a substantially dry coating and/or impregnation of graphite in finely divided and preferably colloidal form.

Oxide-coated aluminum surfaces treated or impregnated with aqueous dispersions of colloidal graphite and dried, as described hereinabove, have good self-lubricating properties which may be retained for considerable periods in service, under some conditions, without the application of any additional binding medium. It therefore appears proper to consider the oxide coating itself as a binding medium in some respects.

However, under adverse conditions, such as are encountered when the coated article is subjected to friction at elevated temperatures in the presence of oil, for example, graphite applied in this manner is loosened and readily removed. In many cases a more permanent retention of the graphite is desirable in order that it may more fully perform the functions for which it was intended, such as improving the wearing qualities of the article and making it permanently self-lubricating. We have discovered that these results can be accomplished by using as the binding medium a combination of an aluminum oxide coating as described hereinabove, for example, and a suitable silicate. For this purpose we choose a soluble silicate, such as a silicate of one of the alkali metals, and we generally employ sodium silicate because of its suitability, availability, and low cost. Furthermore, we prefer to use a relatively non-alkaline silicate having a molecular ratio of silica to sodium oxide higher than 1 to 1, and preferably 2 to 1 or higher, such as sodium disilicate or one of the higher silicates. These are considerably less alkaline than sodium metasilicate, and more compatible with the oxide coating.

In this preferred form of our invention, the silicate may be applied in any suitable manner, but we generally find it to be most convenient to apply it in fluid form, especially when it is applied simultaneously with the graphite dispersion. We have found that although silicates are electrolytes they can be mixed with colloidal graphite dispersions and applied to aluminum articles without apparent detriment to the properties of the graphite. For this procedure, a solution of the selected silicate is made up and mixed with the selected graphite dispersion, which may be prepared as previously described. The combination or mixture of coating and binding materials is then applied to the oxide-coated aluminum article by immersion, rubbing or otherwise. The concentration of the silicate solution can be varied over a range up to its solubility limit, and we have obtained good results by using solutions containing 5 per cent, 10 per cent, and 25 per cent of a sodium silicate having a ratio of silica to soda" of about 3.25 to 1 by weight. The more concentrated solutions containing 20 per cent to 25 per cent silicate, for example, generally seem to make the graphite coating somewhat more permanent, however. The proportions in which the chosen silicate solution is combined with the graphite dispersion may also be varied considerably in accordance with the specific results desired. For example, we have used solutions containing 5 per cent, 10 per cent, and 25 per'cent sodium silicate with approximately equal volumes of graphite dispersions prepared by diluting Aquadag 3.5 to l and 5 to 1 with water, and therefore containing about 3.5 to 6 per cent graphite; we have also used similar silicate solutions with 2 to 4 volumes of similar graphite dispersions; and we have used various mixtures of concentrated Aquadag" and 20 per cent sodium silicate solution.

After drying, all of these mixtures resulted in improved adherence of the graphite to the aluminum article when subjected to friction in the presence of oil, although little or no improvement was noted when using dilute silicate solutions containing about 1 per cent sodium silicate, for example. It therefore appears that good graphite coatings can be produced by applying to oxide-coated aluminum articles a mixture of a solution containing about 5 per cent or more of a sodium silicate having a silica to soda ratio greater than 1 to 1, with about 1 to 4 vol umes or more of substantially aqueous graphite dispersion containing from about 2 per cent to about 22 per cent of finely divided graphite, and drying. Especially good results have been obtained in the treatment of aluminum pistons, for example, by applying to the oxide-coated piston surface a mixture of one part sodium silicate solution containing 25 per cent of a silicate having a ratio of silica to soda of about 3.25 to 1, by weight, with two parts of a dispersion prepared by diluting concentrated Aquadag with 3 volumes of water, and drying. In all cases it is preferable to apply enough graphite dispersion to form a coating on the surface. of the article in addition to whatever amount is adsorbed in the oxide coating.

While we generally prefer to use as the binding medium, for holding the graphite coating on aluminum articles treated in accordance with our invention, the combination of an aluminum oxide coating and a silicate, as described hereinabove, we have also found that in many instances a coating of satisfactory adherence and lubricating and wearing properties can be produced by the use of a silicate alone as the binding medium. In this form of our invention the surface of the aluminum article is cleaned, and may then be etched if desired, although etching is not generally necessary. A suitable mixture of graphite dispersion and silicate solution, as described in the preceding paragraph, for example, is then applied directly to the surface by painting, immer- "51011 or otherwise, and dried. This produces a graphite coating which in many respects is equal to the coatings obtained when using an aluminum oxide coating as described hereinabove.

As one example of an application of our inven tion, we will now describe the treatment of a set of aluminum pistons for a small automobile engine. These pistons were made of a widely used aluminum casting alloy containing silicon. They were first cleaned, then immersed in an electro-- lytic bath containing 15% H2504. There they were connected as anodes and subjected for about 30 minutes to an electrolytic treatment at a current density of 11 to 12 amperes per square foot and an E. M. F. of 23 volts. with the temperature maintained at about F. The surfaces of the pistons were thus provided with an oxide coating of substantial thickness having the usual properties of such coatings and adapted to serve as a binding medium for graphite. The coated pistons were then washed and dried. A mixture was then prepared by making a 25% solution of sodium silicate having a ratio of silica to soda of about 3.25 to 1,- and making a substantially aqueous dispersion of finely divided graphite by diluting concentrated with 3 volumes of water, and mixing one part of the solution with 2 parts of the dispersion. This mixture was painted onto the surfaces of the pistonaallowed to dry, and then heated to C. for one hour to complete the drying. A few days later the pistons were installed in an automobile engine" for testing and observation, and they were then subjected to 65,000 miles of spect to surface appearance and substantially free from the effects of wear and scufling.

While our invention has been described hereinabove with special reference to certain embodiments thereof and practices therein which are now preferred, it is to be understood that the invention may be otherwise embodied and practiced within the scope of the appended claims.

We claim:

1. As a new article of manufacture, an aluminum object having a self-lubricating surface, consisting of an aluminum base and an adherent alkali silicate.

2. As a new article of manufacture, an aluminum object having a self-lubricating surface, consisting of an aluminum base and an adherent lubricating coating comprising finely divided graphite secured to said base by an artificiallyproduced aluminum oxide coating formed on said base and sodium silicate.

3, The method of treating the surface of an aluminum article which comprises forming thereon an artificially-produced, dense, hard, adherent coating of aluminum oxide and applying thereto a mixture of an aqueous dispersion of finely divided graphite and an aqueous solution of an alkali silicate, and drying to produce an adherent, self-lubricating surface bonded to the article by the oxide coating and the alkali silicate.

4. The method of treating the surface of an aluminum article which. comprises forming thereon an artificially-produced, dense, hard, adherent coating of aluminum oxide and applying thereto a mixture of an aqueous dispersion of finely divided graphite and an aqueous solution of sodium silicate having a ratio of silica to sodium oxide greater than 2 to 1 by weight, and drying to produce an adherent, self-lubricating surface bonded to the article by the oxide coating and the sodium silicate.

HAROLD 1;. WORK. CHARLES J. SLUNDER.

CERTIFI GATE OF C ORRECTI 0N Patent No. 2,157,155. m 9; 19 9.

HAROLD K. WORK, ET AL.

It is hereby certified that error appears in the prinized specification of the above numbered patent requiring correction as follows: Page 2, second column, line 56, after the comma 'and before the word "and"'insert 20 per cent; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office. 7

Signed and sealed this 6th day of June, A. D. 1939.

Henry Van Arsdale (Seal) Acting Commissioner of Patente. 

